Noncontact printers which utilize charged droplets are generally known as evidenced by U.S. Pat. Nos. 3,373,437 to Sweet et al; 3,560,988 to Crick; 3,579,721 to Kaltenbach; and 3,596,275 to Sweet. Typically, fluid filaments of e.g. ink, dye, etc. are issued through respective orifices of an orifice plate. An array of individually controllable electrostatic charging electrodes is disposed downstream of the orifice plate along the so-called "droplet formation zone." In accordance with known principles of electrostatic induction, the fluid filament is caused to assume an electrical potential opposite in polarity and related in magnitude to the electrical potential of its respective charging electrode. When a droplet of fluid is separated from the filament, this induced electrostatic charge is trapped on and in the droplet. Thus, subsequent passage of the charged droplet through an electrostatic field having the same polarity as the droplet charge will cause the droplet to be deflected away from a normal droplet path towards a droplet catching structure. Uncharged droplets, on the other hand, proceed along the normal path and are eventually deposited upon a receiving substrate.
A problem arises, however, in that should the charging and/or deflection electrode become inoperative due to, for example, loss or interruption of electrical power or shorting of the electrodes to ground potential, it would become impossible to electrostatically charge the droplets and/or to thereafter deflect the charged droplets from the normal droplet flight path. That is, should either the charging electrode or deflection electrode malfunction, substantially all of the droplets may behave similarly to uncharged droplets and will thus proceed along the normal droplet flight path and be deposited upon the print medium. There exists therefore the possibility that the print medium will become flooded due to the inability of the charging electrode and/or deflection electrode to perform their intended functions. As higher printing speeds are used, great waste of fluid and substrate material can occur as the result of electrode failure over even short intervals of time.
Often, the charge and/or deflection electrodes of an ink jet printer can become electrically shorted to ground potential during the course of normal printing operations as a result of electrical bridging via particulate impurities in the printing fluid and even via the fluid itself. The distance between charging electrodes and fluid filaments is typically very small (on the order of thousandths of an inch) to ensure proper electrostatic charge induction. Consequently, charge electrodes may sometimes become wetted with a quantity of printing fluid. Printing fluid typically has sufficient electrical conductivity to cause current to flow from the wetted charge electrode to any other structure also in contact with the quantity of fluid, thereby decreasing the charging potential of the wetted charge electrode. Small particles in the fluid which become lodged between a charge electrode and a structure at ground potential (such as an electrode mounting fixture, etc.) can completely short the charging electrode to ground potential.
Shorting of charging and/or deflection electrodes can usually be easily cured by simply drying and/or cleaning the shorted electrode (e.g. by using suction). Unfortunately, the method usually used to detect electrode shorts is for an operator to visually monitor the final printed substrate. An inattentive operator can thus cause extreme waste of printing substrate and printing fluid. Moreover, less-than-catastrophic malfunctions of the electrode can cause printing defects which are not readily discernible to the naked eye as the printing substrate is conveyed past an operator after printing but which should nevertheless be avoided to ensure high quality printing.
The present invention overcomes such disadvantages by providing means by which malfunctions in the charging and/or deflection electrodes are automatically detected. In accordance with one aspect of the invention, droplets in an ink jet printing apparatus are captured to prevent the printing substrate from becoming flooded when the charging and/or deflection electrodes have malfunctioned. The auxiliary droplet catching structure of the present invention is mounted for reciprocal rectilinear movements between a retracted position wherein the catching structure is retracted from the generated droplet streams and an advanced position wherein the catching structure intercepts the droplet streams. In such a manner, the droplet catching structure when in the advanced position prevents the droplet streams from proceeding along the droplet flight path and being deposited upon the print medium.
The movements of the auxiliary catching structure between the retracted and advanced positions are controlled so that the structure operates in a "fail safe" manner. Thus, when the control system senses an inability of the charging and/or deflection electrodes to properly charge and/or deflect droplets, respectively, the auxiliary catch pan will be moved into the advanced position so as to prevent flooding of the substrate due to its interception of substantially all droplets along the normal droplet flight path.